[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Karl-Arne Markström]

Feed-forward correction is a very powerful, but somewhat involved
distorsion reduction technique. For more information, 
and some worked examples, see chapter 13  "Ultra-low distorsion power amplifiers" in 
"Single Sideband Systems & Circuits".

Another aspect is to generate a low-power signal with an IM3 level approaching -75 dB.

This may turn out be a major obstacle, as the transmit signal path of the FT1000D will
need to be designed in more or less the same way as the receive path. 

The cleanest SSB exciter I have used was the ITT-Standard Radio TD91, 
while using some alignment "tricks", can provide a - 65 dB IM3 level at 50 mW PEP output.

In the TD91 the signal path is closely related to the CR91 receiver, using high-current U350 quad JFET
ring mixers, and class-A CATV transistors with generous bias levels to increase the signal to the  50 mW level.

When discussing IMD levels lower than about - 60 dB, the distorsion properties of passive components
such as crystal filters and ferrite core tuned circuits also need to be considered.

73/

Karl-Arne
SM0AOM








----- Original Message ----- 
From: "Tom Cathey" <K1JJ at comcast.net>
To: <wc6w at juno.com>; <amps at contesting.com>
Sent: Monday, December 19, 2005 6:32 PM
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback


> Hi Marv,
> 
> Gee, that feed forward correction method sounds like something that would be 
> ideal for the FT-1000D. As you said, it could be later moved from rig to rig 
> too.
> 
> So far all I've seen on the web are ivory tower block diagrams, etc. Is 
> there a tested circuit available somewhere to build up or a commercial 
> surplus unit? Even something that has suggested chip choices with a circuit 
> I could then modify to my use would help.
> 
> For this particular project, I don't mind building up the two stages to go 
> from 5mW to 250mW using a chip and a tube, but this feed forward correction 
> idea could open the door to setting up other clean linear systems from 
> exisiting stand alone linears I have now in the shack..
> 
> Just to double check... it requires TWO inputs:  The 5mW, and  the 1000D 
> 200W module output for sampling, correct?   And the big linear final would 
> have it's own negative feedback loop back to the tube driver, if used, 
> independent of the feed forward system? [assuming we don't use your 200 ohm 
> swamped system and still use a tube to drive the final]
> 
> 
> 73,
> Tom, K1JJ
> 
> --------------------------
> Hi Tom,
>    The FF system would be entirely external to your FT-1KD -- It just 
> connects between the rig's output the outboard linear, plus an input from 
> the low level tap.  Therefore, transportable to a new rig with a low level 
> out -- an easy addition, if necessary.  It could even be left in line with 
> only a slight (1-2%) loss of power when not in use.
> 
>    Regarding the link, which looks to be more of a mechanical diagram than a 
> schematic, it might not be GG.  The final neutralization ckt. resembles a 
> grid driven stage.  The 1st-2nd stage coupling also appears to be more 
> suitable for grid driven operation.  Perhaps, it's 3 grid driven stages? 
> Where have we seen that before...
> 
> 73 & Good evening,
>   Marv WC6W
> 
> 
> 
> 
> 
> 
> 
> 
> 
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